For several years U.S.-made automobiles have been equipped with "two-way" catalytic converters to reduce the level of unburned hydrocarbons and carbon monoxide in the exhaust from automotive gasoline engines. These "two-way" catalytic converters have, however, not had the capability of making significant reductions in the amount of the third major component of automotive exhausts, nitrogen oxides. Under the air pollution regulations which are to come into effect in the near future, automotive exhaust systems must have the capability of reducing the level of nitrogen oxides in the exhaust as well as the unburned hydrocarbons and carbon monoxide levels. To accomplish this reduction of nitrogen oxide level, the automotive industry plans to use catalytic converters containing what are known as "three-way" catalysts, which have the capability of reducing the levels of all three major components in the automotive exhaust systems.
The three-way catalysts, however, have the limitation that they must operate at significantly higher temperatures than the present two-way catalysts. In order to attain this high temperature of the exhaust gases at the inlet to the three-way catalytic converter, the automotive industry has been investigating the use of "light-off" catalysts placed in a container between the outlet of the engine exhaust manifold and the inlet of the three-way catalytic converter. The function of the light-off catalyst is to create an exothermic reaction with the unburned hydrocarbons in the exhaust system to raise the exhaust gases to a temperature at which the three-way catalyst will operate effectively to reduce the level of all three pollutants.
This system including the light-off catalyst is, however, quite expensive, and requires the presence of an additional element--the light-off catalyst--in the system, which adds to the system complexity and expense. It also represents an active element in the exhaust system which can fail and which will at least require periodic maintenance and replenishment of the catalyst. It would therefore be quite advantageous to have an exhaust system which would provide for the needed high inlet temperatures of the exhaust gas at the inlet of the three-way catalytic converter but yet would contain no active components. It would also be advantageous for such a system to be simple in structure and relatively inexpensive.